Method and apparatus for controlling a piezo actuator

ABSTRACT

A method and apparatus for controlling a piezo-electric actuator coupled to a driven member is disclosed. The piezo-electric actuator is responsive to waveforms with asymmetrical voltage/current profiles on the rising and falling edge to effect consistent and cumulative movement of the driven member in one of two directions throughout the reciprocations of the piezo-electric actuator. The waveforms are digitally generated from a stored set of numbers or a function for generating the set of numbers. The numbers correspond with the discrete digital values associated with the desired waveforms for moving the driven member in either of at least two directions. The controller may be used to drive more than one piezo-electric actuator. The controller may include responsiveness to a feedback of the position of the driven member to accurately position the driven member. The controller may also include the ability to update the function or values stored in memory so as to couple more efficiently with new or existing actuators. The controller exhibits a relatively smaller form factor and reduced complexity when compared with prior art analog drivers.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of prior application Ser. No.09/706,369 filed on Nov 3, 2000 now U.S. Pat. No. 6,476,537 and entitled“Method and Apparatus for Controlling A Piezo Actuator” whichapplication claims the benefit of priority to Provisional ApplicationNo. 60/163,329, filed Nov. 3, 1999, entitled “PICO Motor Driver” thedisclosures of which are incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to controllers for electromechanicalactuators and more particularly to controllers for piezoelectricelements.

2. Description of the Related Art

Piezo-electric actuators are used in for positioning elements in a widerange of applications. In optical test and measurement they are used forpositioning of lenses, filters, polarizers, mirrors, radiation sources,detectors or a stage to which any of the aforementioned may be attached.In cameras piezo-electric actuators are used for focusing lenses.

Typically such actuators have opposing ends, one of which is fixed andthe other of which is frictionally coupled with a driven member, e.g.lens. As voltage is applied to the piezo-electric actuator an expansionor contraction of the actuator takes place. Since one end of thepiezo-electric actuator is fixed the expansion or contraction of theactuator causes a corresponding movement of the driven member to whichit is frictionally connected. Above some threshold rate of expansion orcontraction the force of the frictional coupling between the actuatorand the driven member is insufficient to overcome the inertia of thedriven member and there is slippage or in the extreme no movement of thedriven member. By driving the piezo-electric actuator with waveformswith asymmetrical voltage/current profiles on the rising and fallingedge it is possible to effect consistent and cumulative movement of thedriven member in one of two directions throughout the reciprocations ofthe piezo-electric actuator.

U.S. Pat. No. 5,394,049 entitled “Piezoelectric Actuator for OpticalAlignment Screws Cross References to Co-Pending Applications” issued onFeb. 28, 1995 and U.S. Pat. No. 5,410,206 entitled “PiezoelectricActuator for Optical Alignment Screws” issued on Apr. 25, 1995, U.S.Pat. No. 6,092,431 and entitled “Rotary type driving device employingelectromechanical transducer and apparatus provided with the rotary typedriving device” issued on Jul. 25, 2000 and U.S. Pat. No. 6,111,336entitled “Driving apparatus using transducer” issued on Aug. 29, 2000each disclose piezo-electric actuators which exhibit the above discussedprincipals. Each of these references is incorporated by reference as iffully set forth herein.

Each of these references discloses various analog drive mechanisms fordelivering the asymmetrical waveforms to the piezo-electric actuator.These circuits rely on various analog components, e.g resistors,capacitors, current sources and sinks in conjunction with appropriatetransistor switches to deliver the required waveform to thepiezo-electric actuator. There are several problems which such circuitsexhibit. First, they are complex and may require a large form factor.Second, the waveforms they generate are not consistent over time sincethey are generated using the time constants associated with resistorcapacitor combinations or of fixed current sources.

What is needed is a drive circuit with reduced cost and complexity whichdelivers repeatable waveforms with the desired characteristics to thepiezo-electric actuator.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for controlling apiezo-electric actuator coupled to a driven member. The piezo-electricactuator is responsive to waveforms with asymmetrical voltage/currentprofiles on the rising and falling edge to effect consistent andcumulative movement of the driven member in one of two directionsthroughout the reciprocations of the piezo-electric actuator. Thewaveforms are digitally generated from a stored set of numbers or afunction for generating the set of numbers. The numbers correspond withthe discrete digital values associated with the desired waveforms formoving the driven member in either of at least two directions. Thecontroller may be used to drive more than one piezo-electric actuator.The controller may include responsiveness to a feedback of the positionof the driven member to accurately position the driven member. Thecontroller may also include the ability to update the function or valuesstored in memory so as to couple more efficiently with new or existingactuators. The controller exhibits a relatively smaller form factor andreduced complexity when compared with prior art analog drivers.

In an embodiment of the invention a controller for controlling at leastone piezo actuator is disclosed. The piezo actuator is coupledfrictionally with at least one positioning member to move thepositioning member in either of two directions as determined by relativerates of expansion and contraction of the piezo actuator. The controllerincludes a logic for generating digitized pulses and andigital-to-analog (A/D) converter. The logic includes the capability togenerate digitized pulses each with a rising edge and a falling edge.The relative absolute values of the corresponding average slopes of therising edge and the falling edge of each of the digitized pulsescorresponds with a selected direction of movement of the at least onepositioning member. The A/D converter includes an input coupled to thelogic and an output coupled to the at least one piezo actuator. The A/Dconverter converts the digitized pulses at the input to an analogwaveform at the output. This effects the movement of the positioningmember in the selected direction.

In an alternate embodiment of the invention a method for controlling atleast one piezo actuator is disclosed. The method for controllingcomprises the acts of:

generating digitized pulses each with a rising edge and a falling edgeand with relative absolute values of corresponding average slopes of therising edge and the falling edge of each of the digitized pulsescorresponding with a selected direction of movement of the at least onepositioning member;

converting said digitized pulses to an analog waveform; and

driving said at least one positioning member with said analog waveformto move sad at least one positioning member in the selected direction.

Other aspects and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1 is a hardware block diagram of a controller coupled to aplurality of piezo-electric actuators.

FIG. 2 shows an embodiment of the data structures from which the desiredwaveforms generated by the controller may be derived.

FIG. 3 is a signal diagram showing the discrete digital valuescorresponding with the desired waveforms, both after an D/A conversionand a subsequent amplification to the appropriate levels for driving thepiezo-electric actuators.

FIG. 4 is a process flow diagram showing the processes associated withthe operation of an embodiment of the controller.

FIG. 5 is a detailed cross-sectional view of a first of thepiezo-electric actuators shown in FIG. 1.

FIG. 6 is a detailed cross-sectional view of a second of thepiezo-electric actuators shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method and apparatus for controlling apiezo-electric actuator coupled to a driven member. The piezo-electricactuator is responsive to waveforms with asymmetrical voltage/currentprofiles on the rising and falling edge to effect consistent andcumulative movement of the driven member in one of two directionsthroughout the reciprocations of the piezo-electric actuator. Thewaveforms are digitally generated from a stored set of numbers or afunction for generating the set of numbers. The numbers correspond withthe discrete digital values associated with the desired waveforms formoving the driven member in either of at least two directions. Thecontroller may be used to drive more than one piezo-electric actuator.The controller may include responsiveness to a feedback of the positionof the driven member to accurately position the driven member. Thecontroller may also include the ability to update the function or valuesstored in memory so as to couple more efficiently with new or existingactuators. The controller exhibits a relatively smaller form factor andreduced complexity when compared with prior art analog drivers.

FIG. 1 is a hardware block diagram of a controller coupled to aplurality of piezo-electric assemblies. The controller 100 is showncoupled to a linear piezo-electric assembly 102 and a rotarypiezo-electric assembly 104. The controller in this embodiment of theinvention accepts input from a workstation 106. The workstation may beused to select one or both of the linear and rotary assemblies foractivation. The workstation may also be used to input directional, orspeed parameters for either of the piezo-electric assemblies. Theworkstation may also be used as a display device to display relative orabsolute position parameters for the assemblies, as well as othercontrol information.

In alternate embodiments of the invention input to the controller may bein the form of an analog signal the polarity of which indicates thedesired direction of movement of the associated piezo-electric assembly.Alternately a digital input may be provided in which a first bit lineselects the direction of motion and a second bit line the frequency atwhich waveforms are generated by the controller thereby governing thespeed of the driven member of the piezo-electric assembly. In stillanother embodiment of the invention an input may be provided fortriggering a single output waveform of either a forward or reverse type.

The controller 100 includes a processor 110, memory 112, adigital-to-analog converter (DAC) 114, an amplifier 116, a powerconverter 118, a multiplexer 132, and a switchable connection 128 to acurrent sink 130. Collectively the processor and memory form a logic forgenerating digitized pulses. The pulses may be generated by a reading bythe processor of an ordered set of numerical values 122 stored in memory(See FIG. 2) or from a function stored in memory the execution of whichby the processor results in the ordered set of numerical values. Theoperation of the processor may be controlled by program code 120 alsostored in memory. The numerical values are output by the processor tothe input of the DAC 114 where they are converted to a stepped analogwaveform (See FIG. 3). The stepped analog waveform may be filtered andsmoothed as it is passed to the input of the amplifier. The amplifieramplifies the analog waveform from the DAC to a range suitable for theassociated piezo-electric assembly to which it is coupled via themultiplexer 132. Peak voltages of over 100 volts may be required toscale the waveform from the DAC to levels at which the desired motion ofthe driven member of either of the piezo-electric assemblies isachieved. The power supply to the amplifier 116 may include theconverter 118 to boost a low voltage input to the required level forpowering the amplifier. The output waveforms which correspond withopposite directions of motion in the driven members have substantiallyopposing symmetries in their leading and trailing edges (See FIG. 3). Bydriving the piezo-electric actuator with waveforms with asymmetricalvoltage/current profiles on the rising and falling edge it is possibleto effect consistent and cumulative movement of the driven member in oneof two directions throughout the reciprocations of the piezo-electricactuator. When the driven member of the associated piezo-electricassembly is properly positioned switch 128 may be shorted to a currentsink 130 to drain current from the piezo-electric actuator. This effectof this is to arrest further movement of the driven member by keepingthe contraction rate of the piezo-electric actuator in the range atwhich slipping between the actuator and the driven member results.

Linear Piezo-Electric Assembly

The piezo-electric assembly 102 includes an actuator 148 a driven member158, and a frame 146. The actuator has jaw elements (See FIG. 5)positioned about the driven member, e.g. a cylindrical shaft, whichincludes a threaded portion 144 passing through the jaws. The base ofthe piezoelectnc actuator is affixed to the frame 146. For the purposeof this explanation, the inertial characteristics of the driven memberare represented by the flywheel portion 140 at the head of the drivenmember 158. Where closed loop control of the position of the drivenmember is enabled position detector 150 operating with linear encoding142 on the shank of the driven member provides position feedback to thecontroller 100.

When the electrical signal across piezo-electric element 160 is suchthat element extends relative longitudinal movements of jaw elementsoccurs. If there is no slippage between the jaws and driven member 158rotation of the driven member takes place in the direction of arrow 154.As the amplitude of the electrical signal across piezoelectric elementis reduced, contraction occurs, causing relative longitudinal movementof the jaw elements in the opposite direction. Again asswning that noslippage occurs between the jaws and driven member, rotation of drivenmember takes place in the direction of arrow 156. A spring clip 152generates damping force of the opposing jaws on the threaded portion ofthe driven member.

Because of the inertia of the driven member 158, a rapidly rising orfalling electrical signal will induce such rapid movement of the jawelements that slippage between the jaws and the driven member willoccur. The duration of slippage depends on the waveform and amplitude ofthe electrical signal applied across the piezoelectric element 160, aswell as the mechanical characteristics of the system, such as thefrictional engagement between the jaws and driven member, and theinertia of the driven member and other mechanical elements connected toit. Conversely, application of a slowly rising or falling signal acrosspiezoelectric element wili cause a correspondingly slow longitudinalmovement of the jaw elements, and very little or no slippage between thejaws and driven member will take place.

It follows that selective rotation of driven member 158 may be obtainedin either direction 154-156 simply by applying a cyclic electricalsignal having the proper waveform to the piezo-electric element 160.Thus, a waveform having a slowly rising leading edge followed by arapidly falling trailing edge will cause rotation in a first direction.Conversely, a waveform having a rapidly rising leading edge followed bya slowly falling trailing edge will be effective to rotate the drivenmember in the opposite direction.

Rotary Piezo-Electric Assembly

The rotary piezo-electric assembly 104 is a rotary optical stage. Itincludes a piezoelectric element 176 mounted in a piezo-electricactuator 174 which is affixed to the base member 170 to which a drivenmember, e.g. rotary stage 172 is rotatably coupled. An optical elementsuch as a diffraction grating, mirror, polarized, or similar device maybe affixed to the rotary stage. Cut out portions in base member allowthe rotatable stage member to be grasped by hand for manual rotation. Aknurled portion on the top of rotary stage may be used in conjunctionwith scale on the top of rotatable stage member to achieve a coarseinitial position. Where closed loop control of the position of thedriven member is enabled position detector 178 operating with encodingon the rotary stage provides position feedback to the controller 100.

The piezoelectric element 176 has a first end which fits into a receptorportion of the base member 170 and second end which is affixed a drivepad which frictionally couples with the rotary stage. The reciprocatingmotion of the drive pad developed by the piezoelectric element isconverted to rotary motion of the optical stage by moving the drive padrelatively slowly in a first direction such that the coefficient offriction between the drive pad and the rotatable optical stage overcomesthe inertial and rotational friction of the rotatable optical stage,causing the moveable optical stage to rotate slightly. A relativelyrapid rate of motion in a direction opposite the first direction resultsin slippage between the drive pad and the rotary stage thereby avoidingmotion of the rotary stage with respect to the base 170. When therelative rates of motion are reversed the direction, i.e.counter-clockwise or clockwise, of the rotary stage with respect to thebase is reversed as well.

The linear or rotary assemblies discussed above may utilize a variety ofposition sensors, e.g. linear and rotary encoders, to provideabsolute/relative position feedback.

FIG. 2 shows an embodiment of the data structures from which the desiredwaveforms generated by the controller may be derived. In this embodimentof the invention two files 202 and 212 each including an ordered set ofnumbers 204 and 214 respectively are stored in memory 112. The orderedset 204 corresponds with a first pulse the leading edge 206 of which hasa gradual slope and the trailing edge 208 of which has a steep slope.The relative absolute value of the average slope along the trailing edgeexceeds that along the leading edge. A pulse resulting from this orderedsequence will correspond with a first of the two directions of motion ofthe driven member. The ordered set 214 corresponds with a second pulsethe leading edge 216 of which has a steep slope and the trailing edge218 of which has a gradual slope. The relative absolute value of theaverage slope along the trailing edge is less than that along theleading edge. A pulse resulting from this ordered sequence willcorrespond with a second of the two directions of motion of the drivenmember.

In alternate embodiments of the invention a single ordered sequence readin either top-down order or a bottom-up order may be sufficient toeffect a selected one of the two possible directions of motion in thedriven member. In still another embodiment of the invention a pluralityof files may be stored in memory each corresponding to the optimaldigital drive signal for an associated piezo-electric assembly. In stillanother embodiment of the invention the files may be updated with newdigital values to activate a new piezo-electric assembly or to improvethe efficiency of an existing assembly. In still another embodiment ofthe invention the data 122 stored in memory may be a function theexecution of which by the processor 110 results in one or the other ofthe ordered sequences of numbers 204 and 214.

FIG. 3 is a signal diagram showing the waveforms resulting from theordered sequence of numbers after an D/A conversion and a subsequentamplification to the appropriate amplitudes for driving thepiezo-electric actuators. Three pulse sequences are shown. The first twoof these show a low frequency train of pulses 306. The second of theseshow an increase in the frequency of the pulses 306. Pulse 306corresponds to the output of the D/A converter 114 (See FIG. 1)resulting from an input of the ordered sequence of numbers 214 shown inthe previous FIG. 2. Pulse 330 corresponds to the amplified output ofamplifier 116 (See FIG. 1) resulting from an input of pulse 306. Theamount of amplification corresponds with the ratio of the peakamplitudes 340 and 320 corresponding to the outputs of the amplifier andthe DAC respectively. The increase in frequency between pulse train 302compared with pulse train 300 results from a decrease in the interval312 between pulses rather than in a change in the duration of the pulseitself. This decreases the overall period 310 of the waveform withouteffecting the pulse duration. In an embodiment of the invention theduration of the pulse leading edge, peak dwell interval 314, andtrailing edge 316 remains constant as frequency varies. Afteramplification the same durations are found for the leading edge, thepeak dwell time 334 and the trailing edge 336 in the amplified waveform330 as well.

The third of the pulse trains 304 shows high frequency train of pulses308. Pulse 304 corresponds to the output of the D/A converter 114 (SeeFIG. 1) resulting from an input of the ordered sequence of numbers 204shown in the previous FIG. 2 Pulse 360 corresponds to the amplifiedoutput of amplifier 116 (See FIG. 1) resulting from an input of pulse308. The amount of amplification corresponds with the ratio of the peakamplitudes 370 at the output of the amplifier and the peak amplitude ofpulse 308. The duration of the leading edge, the peak dwell time, andthe trailing edge of this pulse also, in an embodiment of the invention,remains substantially constant as the frequency of the pulses isincreased or decreased.

FIG. 4 is a process flow diagram showing the processes associated withthe operation of an embodiment of the controller in which multiplexingof various piezo-electric assemblies is effected. Additionally in thisembodiment closed loop feedback of position is effected along withalterations in the direction of movement and/or the speed of thecorresponding driven member may be effected. The process flow diagramshows one embodiment of the ordering of these processes.

Processing commences at start block 400 in which downloading of newprogram code 120 (See FIG. 1) and/or updated or new control data 122 tothe memory 112 (See FIG. 1) may take place. After initialization of anysoftware or hardware registers etc. processing passes to decision block402. In decision block 402 a determination is made as to whether motionof the driven member currently selected by the multiplexer 132 (SeeFIG. 1) is to start or stop. This determination may be based on thevalue in a register, a user input, or a comparison between a closed loopfeedback of actual position with a desired position for the drivenmember. If a change in the position of the selected one of themultiplexed piezo-electric assemblies is called for control passes todecision process 408. If alternately no further motion for the selectedassembly is required then control passes to stop block 404. In stopblock 404 the piezo-electric actuator is temporarily shorted to acurrent sink 130 via switch 128. This avoids further drift of theselected actuator through a rapid discharge thereof. Then control ispassed to next channel process 406 for the selection of the next of themultiplexed actuators after which control returns to decision block 402.

The processing of the selected one of the channels continues in decisionblock 408, in which a determination is made as to the required speed forthe driven member. The speed of the driven member correlates with thefrequency of pulses output by the amplifier. If a faster speed isrequired control passes to process 410 in which a register with a valuecorresponding to the duration of the idle interval 312 (See FIG. 3)between pulses is decremented. Alternately if a slower speed is calledfor control passes to process 412 in which the same register is upincremented. After either operation on the idle interval registercontrol passes to decision block 414.

In decision block 414 a determination is made as to whether aforward/clockwise or reverse/counterclockwise motion of the selecteddriven member is called for. Depending on the outcome of the decisioncontrol may be passed to process 416 or process 418. In process 416 apointer is set to the appropriate one of the two ordered sequences ofnumbers, e.g. sequence 204 for the selected actuator. Alternately, inprocess 418 the pointer is set to the other of the two orderedsequences, e.g. sequence 214. In an alternate embodiment of theinvention an appropriate function for generating the ordered sequencewould be selected in either of these processes. In still anotherembodiment of the invention the pointer by which the processor 110increments through the file could be initialized at the top or bottom ofa single ordered sequence for subsequent reading in opposing directions.Subsequent to either processes 416 or 418 control passes to process 420.

In processes 420-422 the pointer controlled by the processor 110 (SeeFIG. 1) is incremented through the ordered sequence row by row until theend of the ordered sequence is detected in process 422. Control thenpasses to process 424.

At this point one pulse has been output by the processor to the input ofthe DAC 114 (SEE FIG. 1). Next in processes 424-426 a delay 312 (SEEFIG. 3) of an amount corresponding to the idle interval set in either ofprocesses 410-412 above is injected into the output waveform This delayis the method by which the frequency of the composite waveform resultingfrom the pulses is varied in an embodiment of the invention. At thetermination of the delay control passes to decision block 428 in which adetermination is made as to whether closed loop position feedback andcontrol is enabled. If not control returns directly to decision process402 in which either the next channel is selected or the next pulse forthe currently selected channel is output.

Alternately, where closed loop position feedback is enabled controlpasses to decision process 430. In decision process 430 a determinationis made as to whether the actual position of the selected driven membercorresponds with the desired position. Depending on the outcome of thecomparison control will be passed to either of process blocks 432-434for an appropriate setting of a forward or reverse register. This is theregister read in process 414 discussed above. Control then returns todecision process 402 in which either the next channel is selected or thenext pulse for the currently selected channel is output

FIG. 5 is a detailed cross-sectional view of a first of thepiezo-electric actuators 148 shown in FIG. 1. This actuator includes apiezoelectric element 160 having electrodes 512 and 510 at opposite endswith lead wires electrically connected thereto to allow the analogwaveform output by the amplifier 116 (See FIG. 1) to be applied acrosspiezoelectric element. A first end of the piezoelectric element adjacentelectrode 510 is affixed to the base portion of the actuator frame(body), and an opposite end is affixed to a first movable jaw element504, which co-acts with second movable jaw element 502 to engage adriven member 158 (See FIG. 1) held between the inner faces 528 and 526of the jaws.

Resilient flexure connects base portion and the first movable jawelement to accommodate bi-directional lengthwise longitudinal motion ofpiezoelectric element 160. Such lengthwise motion of element 160 causesa longitudinal reciprocating motion of jaw elements, which in turnimparts a rotational motion to a cylindrical element, such as a threadeddriven member e.g. adjustment screw, held between inner faces of thejaws. A pair of spring retention grooves 522-524 on the opposing outersurfaces of the jaws serve to position and retain a flat clamp spring152, as shown in FIG. 1. This damp increases the pressure of the innerfaces of the jaws against the cylindrical element, such as a threadeddriven member, positioned between them. The actuator frame may befabricated from suitable brass stock by means of conventional wireelector-discharge machining techniques. Flat damp sprint 152 may befashioned from any material having suitable spring and fatiguecharacteristics.

Holes, extending through the actuator frame, are used during fabricationof the actuator to stretch the frame during cementing of thepiezoelectric element 160 so that, after assembly, the piezoelectricelement is under compression. This is done to avoid fracturing the bondbetween the frame and piezoelectric element when an electrical signal isapplied to piezoelectric element.

FIG 6 is a detailed cross-sectional view of the rotary piezo-electricactuators 174 shown in FIG. 1. The stainless steel rotatable stagemember 172 has a complemaitary stainless steel lower member 604 each ofwhich screwingly secure to each other. The rotatable stage memberincludes outer surface threads aligned beneath the outer cylindricaldrive surface 602. Internal threads are located on the walls of anaperture within the rotatable stage member 172 to accommodate an opticor other device. Upper stage member and lower member have beveledbearing races which combine with a complementaly race in the base member170 to provide a high precision, low friction ball bearing for rotationof stage member 172.

An actuator cut-out 600 in base member 170 accommodates a piezoelectriccover and frame element 610. The piezoelectric element 176 has aspherical cap 620 on a first end portion and a brass drive pad 616 on asecond end portion. Spherical cap and drive pad may be affixed topiezoelectric element by suitable adhesive such as epoxy. Bias spring614 fits between drive pad and the base is held in position by springadjustment screw 612. The spherical cap 620 bears against the firstopposing face of the base and allows motion of piezoelectric element 176to accommodate runout of the rotary stage 172. The drive pad 616 has abias spring retention means slot which accepts the tapered end of biasspring 614. The bias spring adjustment screw 612 has a tapered point andengages in the screw mount hole to engage one end of the bias spring.The bias spring is positioned to force drive portion of the drive padinto engagement with the cylindrical drive surface portion 602 ofrotatable stage member and to simultaneously force the spherical cap ofthe piezoelectric element against frame element face.

The piezoelectric element is operative to effect reciprocating motion inthe drive pad. The reciprocating motion of the drive pad developed bythe piezoelectric element is converted to rotary motion of the opticalstage by moving the drive pad relatively slowly in a first directionsuch that the coefficient of friction between the drive pad and therotatable optical stage overcomes the inertial and rotational frictionof the rotatable optical stage, causing the moveable optical stage torotate slightly. The waveform is configured to maintain engagementbetween the drive pad and the rotatable optical stage to incrementallyrotate the optical stage. When the limit of extension of thepiezoelectric element is reached, the electrical drive signal is shiftedto cause rapid movement of the drive pad in a second, opposite directionsuch that the inertial characteristics of the rotatable optical stageprevents the rotatable stage from following the drive pad motion and thedrive pad slips against the rotatable optical stage.

The many features and advantages of the present invention are apparentfrom the written description, and thus, it is intended by the appendedclaims to cover all such features and advantages of the invention.Further, since numerous modifications and changes will readily occur tothose skilled in the art, it is not desired to limit the invention tothe exact construction and operation as illustrated and described.Hence, all suitable modifications and equivalents may be resorted to asfalling within the scope of the invention.

What is claimed is:
 1. A method for controlling at least one piezoactuator coupled frictionally with at least one positioning member tomove the at least one positioning member in either of two directions asdetermined by relative rates of expansion and contraction of the atleast one piezo actuator, and the method for controlling comprising theacts of: storing data corresponding to digitized pulses each with arising edge and a falling edge and with relative absolute values ofcorresponding average slopes of the rising edge and the falling edge ofeach of the digitized pulses corresponding with a selected direction ofmovement of the at least one positioning member; moving the at least onepositioning member in a selected one of the two direction by: readingthe data stored in the storing act; iteratively writing digitized pulseswith relative absolute values of the corresponding average slopes of therising and falling edges of each pulse corresponding with the selectedone of the two directions of movement; converting the iterativelywritten digitized pulses to an analog waveform; and driving the at leastone piezo actuator with the analog waveform thereby to move the at leastone positioning member frictionally coupled with the at least one piezoactuator in the selected one of the two directions.
 2. The method forcontrolling of claim 1, wherein the driving act further comprises:switchably electrically coupling to a selected one of a plurality ofpiezo actuators each coupled frictionally with an associated drivenmember; driving the selected one of the plurality of piezo actuatorswith the analog waveform to move the associated driven member of theselected one of the plurality of piezo actuators in the selecteddirection.
 3. The method for controlling of claim 1 wherein the drivingact further comprises the act of: switchably coupling a current sink tothe at least one piezo actuator to discharge the piezo actuator andarrest movement of the at least one positioning member in the selecteddirection.
 4. The method for controlling of claim 1, wherein the storingact further comprises the act of: storing at least one of an orderedsequence of numbers and a function for generating the ordered sequenceof numbers with the ordered sequence of numbers corresponding with atleast one of the digitized pulses.
 5. The method for controlling ofclaim 1, wherein the data corresponds with an ordered sequence ofnumbers and wherein further the act of iteratively writing furthercomprises the act of: reading the ordered sequence of numbers in aselected one of either of two opposing directions to move the at leastone positioning member in a corresponding one of either of the twodirections.
 6. The method for controlling of claim 1, wherein the dataincludes a first ordered sequence of numbers for moving the at least onepositioning member in a first of the two directions and a second orderedsequence for moving the at least one positioning member in a second ofthe two directions.
 7. The method for controlling of claim 1, whereinthe act of iteratively writing further comprises the acts to increase aspeed of movement of the at least one positioning member of: decreasingan interval between the iterative writing of each of the digitizedpulses; and substantially maintaining a duration of each of thedigitized pulses.
 8. The method for controlling of claim 1, wherein thestoring act further comprises the act of: updating the data stored inthe storing act with updated digitized pulses.